This invention relates in general to anti-lock brake systems and in particular an algorithm for testing the road surface with a small pressure release.
An Anti-lock Brake System (ABS) is often included as standard or optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. One type of ABS controls only the vehicle rear wheel brakes. Such a system is referred to as a RWAL in the following.
A typical prior art RWAL is illustrated at 10 in FIG. 1. As shown in FIG. 1, the RWAL 10 is installed on a vehicle having a hydraulic braking system consisting of a brake pedal 12 coupled to operate a dual reservoir master cylinder 14. When the vehicle operator depresses the brake pedal 12, the master cylinder 14 supplies hydraulic fluid under pressure from a front reservoir 14a through a hydraulic line 16a and from a rear reservoir 14b through a hydraulic line 16b to a conventional combination or proportioning valve 18. The combination valve 18 includes a first output line 18a adapted to supply hydraulic fluid at a first predetermined pressure to actuate a pair of vehicle front wheel brakes 19a and 19b. The combination valve 18 also includes a second output line 18b which supplies hydraulic fluid at a second predetermined pressure to actuate a pair of vehicle rear wheel brakes 20a and 20b. 
The RWAL 10 shown in FIG. 1 utilizes a control valve 21 to selectively control the application of pressure to the rear wheel brakes 20a and 20b when the system is in an anti-lock braking mode. The control valve 21 includes a normally open solenoid valve 22 connected between the line 18b and a line 24 which supplies pressurized brake fluid to the controlled rear wheel brakes 20a and 20b. During an anti-lock braking cycle, the normally open valve 22 isolates the rear wheel brakes 20a and 20b from the master cylinder 14 and is commonly referred to as an isolation valve. The isolation valve 22 also can be selectively opened to increase the pressure at the rear wheel brakes 20a and 20b. 
The control valve 21 also includes a normally closed solenoid valve 26, which is connected between the line 24 and a fluid accumulator 28. The normally closed valve 26 is commonly referred to as a dump valve. The dump valve 26 is selectively opened to reduce the pressure at the rear wheel brakes 20a and 20b by bleeding brake fluid from the rear wheel brakes to the accumulator 28. In the RWAL 10, the master cylinder 14 provides a source of pressurized hydraulic brake fluid during an anti-lock braking cycle, thus eliminating the need for a separate source of pressurized hydraulic fluid, such as a motor driven pump, which is usually included in a four wheel ABS.
The RWAL 10 further includes a computer control module 30 which is electrically connected to a wheel speed sensor 40. The control module 30 can be mounted directly upon the control valve 21 or located remotely therefrom. The control module 30 includes a RWAL microprocessor (not shown) which is programmed to control the RWAL 10 in accordance with a RWAL control algorithm and parameters permanently stored in a Read Only Memory (ROM). The RWAL microprocessor also can access a Random Access Memory (RAM) for temporary storage and retrieval of data. A detailed description of the RWAL illustrated in FIG. 1 is included in U.S. Patent Nos. 4,790,607 and 4,886,322.
During vehicle operation, the microprocessor in the RWAL control module 30 continuously receives speed signals from the wheel speed sensor 40. The RWAL microprocessor monitors the speed signals for potential rear wheel lock-up conditions. When the vehicle brakes are applied and the RWAL microprocessor senses a first rear wheel speed departure, which is indicative of an impending wheel lock-up condition, the RWAL microprocessor is responsive thereto to close the isolation valve 22 to isolate the rear wheel brakes 20a and 20b from the master cylinder 14. The RWAL microprocessor then selectively opens the dump valve 26 to reduce the pressure applied to the rear wheel brakes 20a and 20b and thereby correct the rear wheel speed departure. Once the wheel speed departure has been corrected and the controlled wheel has spun up to the vehicle speed, the microprocessor opens the isolation valve 22 to initiate a second wheel speed departure.
The operation of the RWAL 10 is illustrated by the graphs shown in FIG. 2. The upper solid curve labeled 60 represents the velocity of the rear wheels while the dashed curve labeled 61 represents the vehicle velocity. The operation of the isolation valve 22 and the dump valve 26 is illustrated by the curves labeled 62 and 63, respectively. The lower curve, which is labeled 64, shows the pressure applied to the controlled rear wheel brakes.
During an anti-lock braking cycle, the first and second wheel speed departures are labeled 60a and 60b, respectively. Following correction of the second wheel speed departure, which occurs at time t7, the rear wheel brake pressure is maintained at a constant level Pe. If the vehicle transitions from a low mu to a high mu surface, the braking effort exerted by the rear wheels can be increased. Such a transition is detected when the RWAL microprocessor senses an increased deceleration of the vehicle caused by the uncontrolled front wheel brakes 19a and 19b. Accordingly, it is known to generate a series of reapply pulses 62b which reopen the isolation valve 22. The increased pressure initiates a third wheel speed departure, which is labeled 60c in FIG. 2. At time t10, a dump pulse is generated to open the dump valve 26 to reduce the rear wheel brake pressure to a level Pg to correct the third rear wheel departure. Thereafter, the rear wheel brake pressure is held at the level Pg, which is greater than the previously held level Pe. It is further known to generate a fourth wheel speed departure, which is labeled 60d in FIG. 2, to assure that optimum rear wheel braking is provided. The fourth wheel speed departure results in the rear wheel brake pressure being further increased to Ph which is greater than Pg.
This invention relates to an algorithm for testing the road surface with a small pressure release.
As described above, it is known to cause a pair of rear wheel speed departures in a RWAL, during which the actual rear wheel speed drops below the actual vehicle speed, when a transition of the vehicle from a low to a high road surface mu is detected. A pair of such generated wheel speed departures are shown in FIG. 3A. During anti-lock braking cycles on very low mu surfaces, such as surfaces having a mu which is less than 0.3, it has been observed that the rear vehicle wheels can experience a gradual wheel speed departure while the rear wheel brake pressure is held constant. These wheel speed departures are commonly referred to as xe2x80x9cwheel speed sneakdownxe2x80x9d and are well under the 1.3 g deceleration threshold used to trigger an anti-lock braking cycle. A wheel speed sneakdown condition is illustrated at 70 in FIG. 3B. Because the slowing of the actual wheel speed below the actual vehicle speed under wheel speed sneakdown conditions is similar to the reaction of the rear wheels when the vehicle transitions from a low to high mu road surface, the wheel speed sneak down can be misinterpreted by the RWAL microprocessor as a transition from a low mu road surface to a high mu road surface. Accordingly, the RWAL microprocessor may react to the wheel speed sneak down condition by initiating forced wheel speed departures, as also illustrated in FIG. 3B. However, since the vehicle has not transitioned to a higher mu surface, the forced wheel speed departures would deplete the limited amount of pressurized brake fluid available from the master cylinder without a corresponding increase in braking effort. Accordingly, it would be desirable to verify that the mu of the road surface has actually increased before initiating the wheel speed departures.
The present invention contemplates an anti-lock brake system for a vehicle having at least one rear wheel brake connected to a master cylinder. The anti-lock brake system includes an isolation valve connected between the master cylinder and the controlled wheel brake and a dump valve connected to the rear wheel brake. Additionally, the system has a speed sensor for monitoring the speed of a rear wheel associated with the controlled rear wheel brake. The system further includes a controller electrically coupled to the isolation dump valves and the speed sensor. The controller being operative, upon detecting a potential lock-up condition of the vehicle wheel associated with the controlled wheel brake, to selectively operate the isolation and dump valves to correct the potential wheel lock-up condition. Following correction of said potential lock-up condition, the controller is further operative to selectively open the dump valve and monitor the rear wheel speed to determine whether the vehicle has actually transitioned form a low mu road surface to a high mu road surface.
The speed of the controlled wheel speed is monitored for an absence of acceleration, which is indicative that the vehicle has transitioned from a low mu to a high mu road surface. Upon verification of a low to high mu transition, the controller is operative to cause and correct at least one wheel speed excursion. Following correction of the wheel speed excursion, the braking effort of the controlled rear wheel brake will be greater than before the wheel speed excursion.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.